Spiral freezer with precooler

ABSTRACT

A freezer includes a housing having a space therein for receiving a cryogenic gas, and an inlet and an outlet in communication with the space; a conveyor belt having an outer edge and being arranged for movement through the space for transferring a product from the inlet through to the outlet; a solid longitudinal member disposed in the space adjacent the outer edge for segregating the space into an upper chamber and a lower chamber; and a transfer duct operatively associated with the housing and having a first opening in communication with the upper chamber for receiving the cryogenic gas from the upper chamber and a second opening in communication with the lower chamber for expelling the cryogenic gas into the lower chamber.

BACKGROUND

The present embodiments relate to spiral freezers and related processeswherein a cryogen gas is introduced into the freezer for chilling orfreezing of products such as for example food products.

Exhaust gas from known cryogenic freezing systems is removed as wasteand therefore typically 100% of the energy in the exhaust gas is wasted.Spiral freezing systems operate in an isothermal manner (at a constanttemperature) and therefore, gas exhausted is usually at the operatingtemperature of the spiral freezer. This exhaust gas is usually at atemperature of −80° F. (−62.2° C.) to −120° F. (−84.4° C.).

It would therefore be desirable to use the exhaust gas of a spiralfreezer or other type of freezer to capture gas for additionalrefrigeration for more efficient use of the freezer.

SUMMARY OF THE INVENTION

The present inventive embodiments described below include a precoolerapparatus, which may be integrated with the existing spiral or othertype of freezer, to utilize exhaust gas from the freezer to precool aproduct such as a food product, before entering the main or actualfreezing chamber. Such construction and method provides efficiency gainsfor the freezer, e.g. less nitrogen (N₂) gas is used in the freezerwithout diminishing the freezer's capacity.

The present inventive embodiments can be used with a cryogen such as forexample liquid or gaseous carbon dioxide (CO₂) or nitrogen (N₂).

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present inventive embodiments,reference may be had to the following drawing FIGURE taken inconjunction with the description of the embodiments, of which:

The FIGURE is a partial cross-sectional view of a spiral freezer havinga precooler for the freezer of the present embodiments.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the FIGURE, a spiral freezer with precooler apparatus isshown generally at 10. The freezer 10 includes a housing 12 with achamber 14 arranged therein for receiving a drum 16 for rotationalmovement within the chamber. The drum 16 is rotated about an axle 18 towhich it is mounted; the axle connected to a drive mechanism 20 (such asa motor) mounted external to the housing 12. A conveyor belt 22 havingan outer edge 23 or periphery is arranged for rotational movement in aspiral configuration about the drum 16. The conveyor belt 22 may be ofthe continuous type as shown in the FIGURE. The housing 12 includes aninlet 24 at a 26 side of the housing, and an outlet 28 at another side30 of the housing. The inlet 24 and outlet 28 may optionally be disposedat opposed sides of the housing. The conveyor belt 22 is constructed andarranged with respect to the chamber 14 to introduce products 32, suchas food products, through the inlet 24 in the direction of the arrows 34into the chamber where the products are chilled or frozen for beingremoved from the chamber through the outlet 28.

A baffle 36 is disposed in the chamber 14 for segregating the chamberinto an upper freezing zone 38 above the baffle, while the area belowthe baffle is a lower precooling zone 40. The freezing zone 38 occupiesapproximately seventy percent (70%) of the chamber 14, while theprecooling zone occupies approximately thirty percent (30%) of thechamber, for example.

A cryogen, such as a cryogenic liquid or gas, for example nitrogen (N₂)or carbon dioxide (CO₂), is provided to the chamber 14 through the pipes42,44 as indicated by the arrows 46,48, respectively. Each of the pipes42,44 includes a respective valve 50,52 for controlling introduction ofthe cryogen into the chamber 14. If a cryogen liquid 46,48 is used suchliquid will usually change phase into a gaseous form upon introductioninto the chamber 14. By way of example only reference herein may be to acryogenic gas, due to the phase change.

The conveyor belt 22 can be a mesh or solid construction, and can beformed from plastic, metal or a combination of both.

The housing 12 is provided with a precooling zone exhaust duct 54constructed and arranged for example at the first side 26 proximate theinlet 24, and a freezing zone exhaust duct 56 constructed and arrangedfor example at the other side 30 proximate the outlet 28.

A transfer duct 58 is constructed and arranged with respect to thehousing 12 to transfer the cryogenic gas 46,48 in the freezing zone 38to the precooling zone 40 by circumventing the baffle 36. The baffle 36is of solid construction, i.e. no cryogen gas is permitted to passthrough the baffle. A fan 60 is disposed for rotational movement withinthe transfer duct 58 to draw the cryogenic gas 46,48 from the freezingzone 38 through the transfer duct into the precooling zone 40. Thetransfer duct 58 may be a pipe mounted to the sidewall 30, or may beintegrally formed as part of the sidewall.

The baffle 36 is arranged in the chamber 14 so as not to interfere withthe rotational movement of the drum 16 and the conveyor belt 22, and thecontinuous return arrangement of the belt between the zones 38,40.

A controller 62 is electronically connected as shown by the broken line64 to the valves 50,52 and the fan 60. This arrangement permits thecontroller 62 to signal for the necessary flow rate of the cryogenic gas46,48 to be introduced into the freezing zone 38 of the chamber 14 bycontrolling the openings of the valves and the speed of the fan 60.

The apparatus 10 prevents air or atmosphere external to the housing 12from entering the inlet 24 and the outlet 28 by injecting 100% of thetotal mass flow into the freezing zone 38 and then allowing only 90% ofthe total mass flow to enter the precooling zone 40. There can always bea given flow rate of cryogen into the chamber 14. The flow rate can bedesignated as “X” (not shown in the FIGURE). The controller 62 opens orcloses valves 50,52 to a specific orifice diameter so that the flow rateof cryogen into the apparatus 10 maintains a setpoint temperature in theupper freezing zone 38. Because the actual position of the controlvalves 50,52 is known, and the pressure and temperature of the cryogen46,48 entering through the valves are known, the actually mass flow rateof the cryogen into the apparatus is also known. The controller 62operates the fan 60 to draw a mass flow rate of 0.9×(90% of X). The fan60 is operated by a variable speed motor (not shown), so varying themotor speed is directly proportional to the mass flow rate of gas drawnthrough the transfer duct 58 by the fan. Only 90% of the mass flow isdrawn from the upper freezing zone 38 into the lower pre-cooling zone40, because 10% of the gas must be allowed to exit the system underpressure at the outlet 28 of the apparatus 10. This is to preventexternal warm air from entering the freezer apparatus. The remaining 90%of the mass flow, now in the lower precooling zone 40 below the baffle36 is exhausted from the precooling zone exhaust duct 54 and/or acentral exhaust port (not shown). A signal from the controller 62 whichcontrols the variable speed fan 60 in conjunction with the valve 50,52openings permits the apparatus 10 to maintain the necessary mass volumein the chamber 14. A remaining 10% of the cryogenic gas introduced intothe chamber 14 at the freezing zone 38 can be exhausted through theoutlet 28.

As the fan 60 draws the cryogenic gas 46,48 from the freezing zone 38through the transfer duct 58 into the precooling zone 40, the gas comesin contact with the warmer product 32, which has entered the precoolingzone from the inlet 24, to remove energy from the food product prior toit entering the freezing zone 38. The cryogenic gas provided from thetransfer duct 58 into the precooling zone 40 can now be exhausted at theprecooling zone duct 54 at a significantly warmer temperature(approximately −20° F. (−28.8° C.)), thereby increasing the overallefficiency of the apparatus 10. This is because the food product hasbeen precooled in the precooling zone 40 such that a lesser amount ofthe cryogen gas 46,48 is necessary in the freezing zone 38 in order toreduce the temperature of the food product to that which is needed.

In the Example where the cryogenic gas 46,48 is introduced into thechamber 14 through the pipes 42,44, the gas is at −80° F. (−62.2° C.).There would therefore be a 9%-11% overall cryogen efficiency gained. Ifthe upper freezing zone 38 was operated at −80° F. and the lowerprecooling zone 40 at −20° F., the following calculation is an Examplecomparing a conventional isothermal spiral freezer with the presentembodiment, as an isothermal spiral freezer would operate and exhaustthe gas at −80° F.(−80° F.−(−20° F.) is −60° F.). See the followingExample.

Example Conventional Isothermal Spiral Freezer Exhaust=−80° F.(−62.2°C.) Dual Zone Precooler Spiral Freezer 10 Exhaust 54=−20° F.(−28.8° C.)LN₂ Efficiency of Conventional Isothermal Spiral Freezer:

Assume liquid nitrogen @ 30 psig and at a saturated state entering thefreezer.

LN₂ heat of vaporization=78.8 Btu/lb., therefore

total potential refrigeration=78.8 (Btu/lb)+0.24 (Btu/lb.*°F.)×ABS(−300−(−80)).

(the delta T or ΔT is =ABS(−320°−(−80°))F

˜0.24 Btu/lb.*° F. (specific heat of nitrogen gas).

˜ΔT=ABS(−300−(−80)), where −300° F.=Temp. of liquid nitrogen enteringfreezer, and −80° F.=Exhaust temp of gas exiting freezer.

78.8 Btu/lb.+0.24 (Btu/lb.*° F.)×(220)=131.6 Btu/lb.

LN₂ Efficiency of Dual Zone Precooler Spiral Freezer 10:

Assume liquid nitrogen @ 30 psig and at a saturated state entering thefreezer.

90% of exhaust gas leaves freezer through exhaust duct 54 of precoolerzone 40 at a temperature of −20° F.

10% of exhaust gas leaves freezer through exhaust duct 56 of freezingzone 38 at a temperature of −80° F.

Therefore, total potential refrigeration is:

 = 78.8_((Btu/l b)) + [(0.9)(.24_(Btu/l b * F)ABS(-300 − (-20))] +   [(0.1)(.24_(Btu/l b * F))ABS(-320 − (-80))] = 78.8_((Btu/l b)) + [(0.9)(.24_(Btu/l b * F))(280)] + [(0.1)(.24_(Btu/l b * F))(220)] = 78.8_((Btu/l b)) + 65.8_((Btu/l b)) = 144.6_(Btu/l b.)144.6_(Btu/l b.)/131.6_(Btu/l b.) = 1.098  or  9.8%  Cryogen  Efficiency  Gain

The 9.8% represents the overall increase in the capacity of the cryogenused in the apparatus 10 to absorb heat. This is referred to as thecryogen efficiency. Therefore, for the same mass flow rate of cryogenused in a conventional isothermal spiral freezer and in the dual zonefreezer apparatus 10, the present apparatus 10 provides for thecryogenic gas 46,48 to remove 9.8% more heat from the apparatus.

As more cryogen gas 46,48 is introduced into the upper freezing zone 38,the controller 62 will increase the speed of the fan 60 which willincrease the mass flow of cryogen into the lower precooling zone 40. Thefan 60 is controlled by the controller 62 to pull or draw the cryogenicgas at a higher volumetric flow rate.

The baffle 36, in conjunction with the transfer duct 58, prevents thegas 46,48 in the freezing zone 38 from indiscriminately entering theprecooling zone 40, by directing the gas to and through the duct 58 in acontrolled flow depending upon the temperature to be used in theprecooling zone.

It will be understood that the embodiments described herein are merelyexemplary, and that one skilled in the art may make variations andmodifications without departing from the spirit and scope of theinvention. All such variations and modifications are intended to beincluded within the scope of the invention as described and claimedherein. Further, all embodiments disclosed are not necessarily in thealternative, as various embodiments of the invention may be combined toprovide the desired result.

What is claimed is:
 1. A freezer apparatus, comprising: a housing havinga space therein for receiving a cryogenic gas, and an inlet and anoutlet in communication with the space; a conveyor belt having an outeredge and being arranged for movement through the space for transferringa product from the inlet through to the outlet; a solid longitudinalmember disposed in the space adjacent the outer edge of the conveyorbelt for segregating the space into an upper chamber and a lowerchamber; and a transfer duct operatively associated with the housing andhaving a first opening in communication with the upper chamber forreceiving the cryogenic gas from the upper chamber and a second openingin communication with the lower chamber for expelling the cryogenic gasinto the lower chamber.
 2. The freezer apparatus of claim 1, furthercomprising a fan disposed in the transfer duct for moving the cryogenicgas from the upper chamber through the transfer duct into the lowerchamber.
 3. The freezer apparatus of claim 2, further comprising atleast one passageway in communication with the upper chamber forintroducing the cryogenic gas into the upper chamber, and a valveinterposed in the at least one passageway for regulating flow of thecryogenic gas to the upper chamber.
 4. The freezer apparatus of claim 3,further comprising a controller in communication with the valve and thefan for generating a signal to control the valve and the fan to regulatean amount of the cryogenic gas introduced into the upper chamber and aflow rate of the cryogenic gas.
 5. The freezer apparatus of claim 1,further comprising a drum disposed in the space and having an exteriorsurface around which the conveyor belt moves.
 6. The freezer apparatusof claim 1, wherein the conveyor belt comprises a surface area selectedfrom the group consisting of a solid surface and a mesh surface.
 7. Thefreezer apparatus of claim 1, wherein the transfer duct comprises a pipemounted to a sidewall of the housing or optionally formed integral withthe sidewall.
 8. The freezer apparatus of claim 1, further comprising afirst exhaust in communication with the lower chamber, and a secondexhaust in communication with the upper chamber.
 9. The freezerapparatus of claim 1, wherein the conveyor belt is arranged in acontinuous loop.
 10. The freezer apparatus of claim 1, wherein apressure of an upper atmosphere in the upper chamber and anotherpressure of a lower atmosphere in the lower chamber are greater than anambient pressure external to the housing.
 11. The freezer apparatus ofclaim 1, wherein the cryogenic gas comprises at least one of nitrogen orcarbon dioxide.